It is known that hydrogen silanes can be reacted, for example, with allyl chloride in the presence of homogeneous or heterogeneous platinum catalysts to form 3-chloropropylsilanes. This reaction is generally referred to as hydrosilylation (see, for example, equation I). EQU Cl--CH.sub.2 --CH.dbd.CH.sub.2 +HSiCl.sub.3.fwdarw.Cl--CH.sub.2 --CH.sub.2 --CH.sub.2 --SiCl.sub.3 (I)
The process is termed homogeneous hydrosilylation when soluble platinum compounds, in the simplest case, for example, H.sub.2 PtCl.sub.6. 6H.sub.2 O, are used as catalysts (cf. DE-OS 28 51 456, CS-PS 176 910, U.S. Pat. No. 4,292,433, U.S. Pat. No. 4,292,434, DE-AS 11 87 240, DE-PS 11 65 028); heterogeneous hydrosilylations utilize elemental platinum or platinum compounds on a support (cf. U.S. Pat. No. 2,637,738, DE-PS 20 12 229, DE-PS 28 15 316).
It is also known that in the reaction of, for example, allyl chloride with hydrogen silanes to form 3-chloropropylsilanes, a portion of the allyl chloride used reacts with the hydrogen silane in a side reaction with the formation of propylene and of the chlorosilane corresponding to the respective hydrogen silane (see, for example, equation II). EQU Cl--CH.sub.2 --CH.dbd.CH.sub.2 +HSiCl.sub.3.fwdarw.CH.sub.3 --CH.dbd.CH.sub.2 +SiCl.sub.4 (II)
Thus, for example, in the reaction of allyl chloride with trichlorosilane, 25-30 mol. % of the allyl chloride entering into the reaction is converted by this side reaction into propylene, accompanied by the formation of equivalent quantities of silicon tetrachloride. The molar ratio of chloropropylsilane formed to silicon tetrachloride in the crude product is a measure of the selectivity of the reaction and typically attains values of between 2.33:1 (70% yield, based on allyl chloride) and 3:1 (75% yield). It is also known that the formation of propylene can be lessened by a special reaction procedure in pressurized apparatus. However, the result of this procedure is that the propylene obtained in the side reaction undergoes a further quantitative reaction with the hydrogen silane used, with the formation of propylsilanes. Even in the reactions carried out in the conventional manner under normal pressure, the propylene originating from the side reaction largely enters into a further side reaction with hydrogen silane to form the corresponding propylsilanes (cf. also DE 34 04 703 C) (see, for example, equation 3). EQU CH.sub.3 --CH.dbd.CH.sub.2 +HSiCl.sub.3.fwdarw.CH.sub.3 --CH.sub.2 --CH.sub.2 --SiCl.sub.3 (III)
Thus, for example, in an industrial plant, in a heterogeneously catalyzed reaction of allyl chloride and trichlorosilane in a column packed with platinized activated carbon, up to 230 kg propyltrichlorosilane is obtained per 1000 kg 3-chloropropyltrichlorosilane, which indicates an additional requirement for approximately 28% of trichlorosilane, based on the quantity of trichlorosilane which has entered the target product (cf. also DE 41 19 994 A1).
Apart from the additional requirement for hydrogen silane, the problem with such processes is also the complicated separation of the unwanted propylsilanes, for which there are scarcely any other fields of use and which consequently have to be disposed of by expensive methods.